Electronic quasi-duplex communication units use holes in their transmissions to allow other radios receiving those transmissions to interrupt the transmissions, thus obtaining quasi-duplex communication. The interrupting communication unit must send a (high speed data) request packet at the proper time (i.e., when the other radio is "listening"). Therefore, electronic duplex radio communication systems require transmission of synchronization or timing signals informing the receiving radios the location of those holes to enable them to interrupt at the proper time. This is done with low-speed signaling. Moreover, it is desirable for electronic duplex radios to be compatible with existing simplex communication systems. The simplex communication systems include a plurality of simplex communication units with sequential, as opposed to simultaneous, talk and listen capability. In simplex systems, the communication units are generally grouped with each other such that communications directed at one group is not heard by others in different groups. Such selective communication capability is achieved by sub-audible coded squelch signals. Such coded squelch signals could comprise analog sub-audible tone coded squelch signals or digital coded squelch signals.
Quasi-duplex communication units may not operate in a simplex communication system because such system does not provide hole timing information to enable the communication units perform their interruption routines for establishing a quasi-duplex communication. On the other hand, the quasi-duplex communication systems do not provide the coded squelch capability needed for selective communication among the simplex communication units. It is, therefore, desired to provide communication capability among quasi-duplex communication units and the simplex communication units in an integrated communication system.